The sequences of both DNA strands are typically determined and compared as a check on accuracy. The sequence of the second strand will be the reverse complement of the first, which makes direct comparison tedious. Arrangement of the four reactions in the order shown (GTAC) allows the base sequence of the complementary strand on a second autoradiogram to be read directly in the same order as the first strand, by turning the autoradiogram over, and reading from top to bottom (think about it!).
Homework: Read the sequence of this DNA ladder. Sequencing labs in Bio2250 was conducted like this, < 1991.